Method of fabricating nonpolar gallium nitride-based semiconductor layer, nonpolar semiconductor device, and method of fabricating the same

ABSTRACT

A method of fabricating a nonpolar gallium nitride-based semiconductor layer is provided. The method is a method of fabricating a nonpolar gallium nitride layer using metal organic chemical vapor deposition, and includes disposing a gallium nitride substrate with an m-plane growth surface within a chamber, raising a substrate temperature to a GaN growth temperature by heating the substrate, and growing a gallium nitride layer on the gallium nitride substrate by supplying a Ga source gas, an N source gas, and an ambient gas into the chamber at the growth temperature. The supplied ambient gas contains N 2  and does not contain H 2 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of Korean Patent Application No.10-2012-0028589, filed on Mar. 21, 2012, and Korean Patent ApplicationNo. 10-2012-0029906, filed on Mar. 23, 2012, which are therebyincorporated by reference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Exemplary embodiments of the present invention relate to a galliumnitride-based semiconductor device, and more particularly, to a methodof fabricating a nonpolar gallium nitride-based semiconductor layer, anonpolar semiconductor device, and a method of fabricating the same.

2. Discussion of the Background

Gallium nitride-based compounds are recognized as important materialsfor high-power high-performance optical devices or electronic devices.In particular, since group-III nitrides, such as GaN, have excellentthermal stability and a direct transition energy band structure,group-III nitrides have recently attracted much attention as materialsfor light emitting devices of a visible ray region and an ultravioletray region. For example, blue and green light emitting devices usingInGaN have been utilized in a variety of applications, for example,large-sized natural-color flat panel display devices, traffic lights,indoor illumination, high-density light sources, high-resolution outputsystems, and optical communications.

However, since if is difficult to fabricate homogenous substratescapable of growing the group-III nitride semiconductor layers thereon,group-III nitride semiconductor layers have been grown on heterogeneoussubstrates having a similar crystal structure through metal organicchemical vapor deposition (MOCVD). As the heterogeneous substrates,sapphire substrates with a hexagonal structure have been mainly used. Inparticular, since GaN epitaxial layers tend to be grown with a c-planeorientation, sapphire substrates with a c-plane growth surface have beenmainly used.

However, an epitaxial layer grown on a heterogeneous substrate has arelatively high dislocation density due to lattice mismatch and thermalexpansion coefficient difference with respect to a growth substrate. Itis known that an epitaxial layer grown on a sapphire substrate generallyhas a dislocation density of IE8/cm² or more. Such an epitaxial layerhaving a high dislocation density has a limit to improving the luminousefficiency of light emitting diodes.

Furthermore, a c-plane gallium nitride-based semiconductor layer grownon a c-plane growth surface generates an internal electric field due tospontaneous polarization and piezoelectric polarization, which reduces aradiative recombination rate. In order to prevent such polarizationphenomenon, research into nonpolar or semipolar gallium nitride-basedsemiconductor layers is in progress. As one of such research, attemptshave been made to form a gallium nitride layer using a nonpolar orsemipolar gallium nitride substrate as a growth substrate. However, in acase where a gallium nitride layer is grown on a nonpolar galliumnitride substrate using a growth method on a sapphire substrate, thegallium nitride layer has a very rough surface morphology. In a casewhere a semiconductor device, such as a light emitting diode, isfabricated using such a gallium nitride layer, a leakage current islarge and a nonradiative recombination rate is increased, making itdifficult to obtain excellent luminous efficiency.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention provide a method ofgrowing a nonpolar gallium nitride-based semiconductor layer enable ofimproving surface morphology of a gallium nitride-based semiconductorlayer which is grown on a nonpolar gallium nitride substrate.

Exemplary embodiments of the present invention also provide a method offabricating a semiconductor device by forming nonpolar galliumnitride-based semiconductor layers with high crystal quality on anonpolar gallium nitride substrate.

Exemplary embodiments of the present invention also provide asemiconductor device having a nonpolar or semipolar semiconductor layerwith improved crystal quality.

Exemplary embodiments of the present invention also provide a nonpolaror semipolar light emitting diode and a method for fabricating the same.

In a case where the gallium nitride layer is grown on the conventionalsapphire substrate, H₂ or both of H₂ and N₂ are mainly used as theambient gas. Since H₂ has high molecular mobility, it is advantageous toequalize the internal temperature of the chamber. Furthermore, since H₂functions to clean the surface of the sapphire substrate, it ispreferably used to grow the gallium nitride layer with excellent surfacecharacteristics. However, if the gallium nitride layer is grown on thenonpolar gallium nitride substrate using the same condition as thecondition of growing the gallium nitride layer on the sapphiresubstrate, the gallium nitride layer with a rough surface is grown. Theinventors of the present application found that anisotropy on thesurface of the nonpolar gallium nitride substrate affected the surfacemorphology, and have accomplished the present invention.

An embodiment of the present invention provides a method of fabricatinga nonpolar gallium nitride layer using metal organic chemical vapordeposition. This method includes: disposing a gallium nitride substratewith an m-plane growth surface within a chamber; raising a substratetemperature to a GaN growth temperature by heating the substrate; andgrowing a gallium nitride layer on the gallium nitride substrate bysupplying a Ga source gas, an N source gas, and an ambient gas into thechamber at the growth temperature. The supplied ambient gas contains N₂and does not contain H₂.

H₂ gas etches the surface of the gallium nitride substrate. However, dueto the anisotropy of the gallium nitride substrate, the surface of thegallium nitride substrate is differently etched according to directionsby H₂ gas. In addition, since the gallium nitride layer grown on thesubstrate is anisotropically etched by H₂ gas, the gallium nitride layeris grown to have a rough surface. Therefore, according to the presentinvention, by cutting off the supply of H₂ gas used as the ambient gas,it is possible to prevent the substrate surface or the growing GaN frombeing etched by H₂ gas. Consequently, the gallium nitride layer withimproved surface morphology can be fabricated.

Furthermore, N₂ gas alone may be supplied into the chamber as theambient gas.

On the other band, the Ga source gas may be TMG or TEG, and the N sourcegas may be NH₃.

The GaN growth temperature may be equal to or higher than 950° C. andlower than 1,050° C. In particular, the GaN growth temperature may be1,000° C.

In addition, the N source gas and the ambient gas may be supplied evenwhen the substrate temperature is being raised to the GaN growthtemperature.

The Ga source gas may be supplied even when the substrate temperature isbeing raised. However, in order to prevent GaN from being formed at anunstable temperature, the Ga source gas may be supplied only at the GaNgrowth temperature. In addition, the gallium nitride substrate may bemaintained for 3 to 10 minutes after the gallium nitride substratereaches the GaN growth temperature and before the Ga source gas issupplied.

An embodiment of the present invention provides a method of fabricatinga semiconductor device by using the nonpolar gallium nitride layerfabricated using the above-described method.

An embodiment of the present invention provides a semiconductor deviceincluding: a gallium nitride substrate; a gallium nitride-based firstsemiconductor layer disposed on the gallium nitride substrate; and anintermediate-temperature buffer layer disposed between the galliumnitride substrate and the first semiconductor layer. Theintermediate-temperature buffer layer is grown on the gallium nitridesubstrate in a growth temperature of 700 to 800° C.

The gallium nitride substrate has a nonpolar or semipolar growthsurface, and the intermediate-temperature buffer layer is directlyformed on the growth surface of the gallium nitride substrate. Inparticular, the gallium nitride substrate may have an m-planegrowth-surface, and the intermediate-temperature buffer layer may bedisposed on the m-plane growth surface. On the other hand, theintermediate-temperature buffer layer may be a GaN layer.

The semiconductor device may further include: a second semiconductorlayer disposed above the first semiconductor layer; and an active layerdisposed between the first semiconductor layer and the secondsemiconductor layer. The semiconductor device may be a nonpolar orsemipolar light emitting diode.

An embodiment of the present invention provides a method of fabricatinga semiconductor device including: forming an intermediate-temperaturebuffer layer on a gallium nitride substrate in a range of 700 to 800°C.; and growing a gallium nitride-based first semiconductor layer on theintermediate-temperature buffer layer at a growth temperature higherthan a formation temperature of the intermediate-temperature bufferlayer.

The gallium nitride substrate may have a nonpolar or semipolar growthsurface. In particular, the gallium nitride substrate may have anm-plane growth surface, and the intermediate-temperature buffer layermay be formed on the m-plane growth surface. Furthermore, theintermediate-temperature buffer layer may be formed of GaN.

The growing of the first semiconductor layer may include: stopping asupply of a gallium source after the intermediate-temperature bufferlayer is formed; raising a temperature of the gallium nitride substrateto a growth temperature of the first semiconductor layer; maintainingthe gallium nitride substrate at the growth temperature of the firstsemiconductor layer for 3 to 10 minutes; and resuming the supply of thegallium source to grow a gallium nitride-based layer on theintermediate-temperature buffer layer.

The method may further include: growing an active layer on the firstsemiconductor layer; and growing a gallium nitride-based secondsemiconductor layer on the active layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a schematic flow diagram for describing a method offabricating a nonpolar gallium nitride layer according to an embodimentof the present invention.

FIG. 2 is a schematic cross-sectional view for describing the method offabricating the nonpolar gallium nitride layer according to theembodiment of the present invention.

FIG. 3 is a temperature profile for describing the method of fabricatingthe nonpolar gallium nitride layer according to the embodiment of thepresent invention.

FIGS. 4A to 4C are optical photographs showing surface morphologies ofthe nonpolar gallium nitride layer according to kinds of ambient gas.

FIGS. 5A to 5F are optical photographs snowing surface morphologies ofthe nonpolar gallium nitride layer according to a growth temperature.

FIG. 6 is a cross-sectional view for describing a light emitting diodeaccording to another embodiment of the present invention.

FIG. 7 is a cross-sectional view for describing a light emitting diodeaccording to another embodiment of the present invention.

FIG. 8 is a graph for describing a method of growing anintermediate-temperature buffer layer and a semiconductor layeraccording to another embodiment of the present invention.

FIGS. 9 and 10 are cross-sectional views for describing a superlatticelayer of a light emitting diode according to another embodiment of thepresent invention.

FIG. 11 is a cross-sectional view for describing an active layeraccording to another embodiment of the present invention.

FIG. 12 illustrates an energy band for describing the active layer ofFIG. 11.

FIGS. 13A to 13C are optical photographs for describing surfacemorphologies of a gallium nitride layer according to another embodimentof the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The invention is described more fully hereinafter with reference to theaccompanying drawings, in which exemplary embodiments of the inventionare shown. This invention may, however, be embodied in many differentforms and should not be construed as limited to the exemplaryembodiments set forth herein. Rather, these exemplary embodiments areprovided so that this disclosure is thorough, and will fully convey thescope of the invention to those skilled in the art. In the drawings, thesize and relative sizes of layers and regions may be exaggerated forclarity. Like reference numerals in the drawings denote like elements.

FIG. 1 is a schematic flow diagram for describing a method offabricating a nonpolar gallium nitride layer according to an embodimentof the present invention, FIG. 2 is a schematic cross-sectional view fordescribing the method of fabricating the nonpolar gallium nitride layeraccording to the embodiment of the present invention, and FIG. 3 is atemperature profile for describing the method of fabricating thenonpolar gallium nitride layer according to the embodiment of thepresent invention. Ambient gas also is illustrated in FIG. 3.

Referring to FIGS. 1, 2 and 3, a nonpolar gallium nitride substrate 11is disposed inside a MOCVD chamber (S1). In this embodiment, thenonpolar gallium nitride substrate 11 has an m-plane growth surface. Inaddition, the growth surface of the gallium nitride substrate 11 mayhave a tilt angle for assisting the growth of an epitaxial layer.

The gallium nitride substrate 11 having the m-plane growth surface maybe, but is not limited to, a bare substrate fabricated using an HVPEtechnique or the like. The m-plane gallium nitride substrate 11 mayinclude a gallium nitride-based semiconductor layer which is formedthereon and has an m-plane growth surface grown on the bare substrate.For example, a GaN layer or an InGaN layer may be grown on a bare-statem-plane gallium nitride substrate at a relatively low temperature, and anonpolar gallium, nitride layer 13 according to the embodiment may begrown on the GaN layer or the InGaN layer.

The MOCVD chamber is a well-known chamber for growth of galliumnitride-based semiconductor layers. The MOCVD chamber includes a tray onwinch the substrate 11 is to be disposed, and a susceptor on which thetray is positioned. The susceptor is heated by a heater, and heat fromthe susceptor is transferred to the substrate 11 so that the substrate11 is heated. A temperature of the susceptor is measured by athermocouple, and the measured temperature of the susceptor isconsidered as a temperature of the substrate 11. On the other hand, theMOCVD chamber includes a gas supply unit configured to supply an ambientgas (or carrier gas) such as N₂, H₂, or Ar together with a source gas ofgroup III element such as Al, In, or Ga, an N source gas such as NH₃,and an impurity source gas such as SiH₄ or Cp₂Mg.

The susceptor is heated by the heater, and the heat from the susceptoris transferred so that the substrate is heated (S2). Accordingly, asillustrated in FIG. 3, the temperature of the substrate 11 rises from Tato Tb during a period of time from t1 to t2. The temperature of thesubstrate 11 may rise, for example, at a rate of about 100° C./min.

N₂ gas may be supplied as the ambient gas during the heating of thesubstrate 11, and NH₃ may be supplied. It is preferable that H₂ gas isnot supplied during the heating of the substrate 11. In particular, in acase where H₂ gas is supplied when the temperature of the substrate 11is high, for example, above about 800° C., the surface of the substrate11 may be etched by the gas. In this case, since the m-plane galliumnitride substrate 11 has high anisotropy, the surface etching by the H₂gas appears differently depending on a direction on the surface of thesubstrate 11. As a result, the substrate 11 has a stripe-shaped roughsurface. Therefore, it is preferable to cut off the supply of the H₂ gasin the process of raising the temperature of the substrate 11 so as toprevent the surface of the substrate 11 from being rough.

On the other hand, a Ga source gas may be supplied during the heating ofthe substrate 11, but a GaN layer may be grown on the substrate 11 bythe supply of the Ga source gas. Since the GaN layer is formed in astate in which the temperature is not stabilized, the surface of the GaNlayer is readily formed roughly. It is necessary to precisely control aprocess condition so as to grow the GaN layer with excellent surfacemorphology. Therefore, it is preferable that the Ga source gas is notsupplied during the heating of the substrate 11 so as to achieve astable process.

On the other hand, an NH₃ gas supplies nitrogen (N) to the surface ofthe substrate 11, which prevents nitrogen atoms from being separatedfrom the gallium nitride substrate 11. In addition, N₂ as the ambientgas serves to raise the internal temperature of the chamber. Instead ofN₂ or in addition to N₂, inert gas such as Ar may be supplied as theambient gas.

Subsequently, after the temperature of the substrate 11 reaches thegrowth temperature Tb, the substrate 11 is maintained at the growthtemperature Tb during a period of time from t2 to t3 (S3). The period oftime may be, for example, 3 to 10 minutes. The growth temperature Tb maybe in a range from 950° C. to 1,050° C., in particular, about 1,000%° C.During this period of time, N₂ and NH₃ are continuously supplied so thatthe gas state inside the chamber and the surface state of the substrate11 are stabilized. In addition, even during this period of time, H₂ isnot supplied for the same reason as described above. Since the step S3is required for stabilizing the process, the step S3 may be skipped.

Subsequently, a Ga source gas is supplied to the chamber, and thus, agallium nitride layer 13 is grown on the substrate 11 (S4). The galliumnitride layer 13 is grown with an m-plane growth surface along thegrowth surface of the substrate 11. At this time, in a case where H₂ issupplied as the ambient gas, H₂ anisotropically etches the growing GaN.Hence, the surface of the gallium nitride layer 13 may become rough. Forthis reason, H₂ is not supplied as the ambient gas. As the ambient gas,as described above, N₂ may be used, or inert gas such as Ar may be used.In order to dope impurity into the gallium nitride layer 13, an impuritysource gas such as Cp₂Mg may be supplied together.

Experimental Example 1

FIGS. 4A to 4C are optical photographs showing surface morphologies ofthe nonpolar gallium nitride layer 13 according to kinds of ambient gas.Specifically, FIG. 4A shows the case where N₂ is supplied as the ambientgas, FIG. 4B shows the case where H₂ is supplied as the ambient gas, andFIG. 4C shows the case where N₂ and H₂ are supplied together as theambient gas. In all the cases, the gallium nitride layer was grown onthe m-plane gallium nitride substrate 11 at the same growth temperature,and flow rates of source gases were adjusted appropriately to eachambient gas.

As shown in FIG. 4A, in a case where H₂ was not supplied and N₂ wassupplied as the ambient gas, the gallium nitride layer 13 withrelatively excellent surface morphology was grown. On the other hand, asshown in FIGS. 4B and 4C, in a case where H₂ was contained in theambient gas, the surface of the gallium nitride layer was very rough. Inaddition, in a case where only H₂ was supplied, the surface of thegallium nitride layer was most rough as shown in FIG. 4B.

In a case where H₂ is supplied as the ambient gas, it is determined thatthe surface of the gallium nitride layer 13 is formed roughly becausethe surface of the gallium nitride substrate 11 and the gallium nitridelayer are anisotropically etched by H₂.

FIGS. 5A to 5F are optical photographs showing surface morphologies ofthe nonpolar gallium nitride layer according to a growth temperature.The gallium nitride layer of each sample was grown on the m-planegallium nitride substrate 11 under the same condition, except for adifferent growth temperature, and only N₂ gas was used as the ambientgas.

Referring to FIG. 5, the surface morphology of the gallium nitride layer13 is greatly different according to the substrate temperature Tb. Inparticular, it can be seen that the gallium nitride layer withrelatively excellent surface morphology was grown when the substratetemperature was 1,000° C.

FIG. 6 is a cross-sectional view for describing a light emitting diodeaccording to an embodiment of the present invention.

Referring to FIG. 6, the light emitting diode includes a gallium nitridesubstrate 11, a first contact layer 13, a superlattice layer 15, anactive layer 17, a p-type clad layer 19, and a second contact layer 21.In addition, the light emitting diode may further include a transparentconductive layer 23, a first electrode 25, and a second electrode 27.

Since the gallium nitride substrate 11 is the same as described withreference to FIG. 2, the detailed description thereof will be omitted.The first contact layer 13 may be formed of Si-doped GaN. The firstcontact layer 13 is substantially identical to the gallium nitride layer13 of FIG. 2, except that Si is doped as impurity. As described abovewith reference to FIGS. 1 to 3, the first contact layer 13 is grown onthe gallium nitride substrate 11, and N₂ used as the ambient gas,whereas H₂ is not used.

The superlattice layer 15 with a multi-layered structure may be disposedon the first contact layer 13. The superlattice layer 15 is disposedbetween the first contact layer 13 and the active layer 17. Thesuperlattice layer 15 may be formed by repetitively laminating a pair ofInGaN/GaN at multiple periods (for example, 15 to 20 periods), but isnot limited thereto. For example, the superlattice layer 15 may beconfigured in such a way that three-layer structure of InGaN layer/AlGaNlayer/GaN layer is repetitively laminated at multiple periods (forexample, 10 to 20 periods). It is preferable that H₂ gas is not suppliedwhen the superlattice layer 15 is being formed.

On the other hand, the active layer 17 with a multiple quantum wellstructure is disposed on the superlattice layer 15. The active layer 17has a structure in which barrier layers and well layers are alternatelylaminated. For example, the barrier layer may be GaN, AlGaN, or AlInGaN,and the well layer may be InGaN or GaN. The active layer 17 is grown ata relatively lower temperature than the growth temperature of the firstcontact layer 13, but it is preferable that H₂ gas is not supplied.

The p-type clad layer 19 is disposed on the active layer 17, and may beformed of AlGaN or AlInGaN. Alternatively, the p-type clad layer 19 mayalso be formed in a superlattice structure in which InGaN/AlGaN isrepetitively laminated. The p-type clad layer 19 is an electron blockinglayer and prevents electrons from moving to the p-type contact layer 21,leading to an improvement in luminous efficiency. The p-type clad layer19 is grown at a relatively higher temperature than the growthtemperature of the active layer 17. Therefore, it is necessary to raisethe substrate temperature so as to grow the p-type clad layer 19. Atthis time, H₂ gas is not supplied when the substrate temperature isbeing raised. In addition, H₂ gas is not also supplied when the p-typeclad layer 19 is being grown.

On the other hand, the second contact layer 21 may be formed of Mg-dopedGaN. The second contact layer 21 may be disposed on the p-type cladlayer 19. The second contact layer 21 may be grown at a relatively lowertemperature than the p-type clad layer 19. Thus, the step of loweringthe substrate temperature may be required. In the step of lowering thesubstrate temperature or when the second contact layer 21 is beinggrown, H₂ gas is not supplied as the ambient gas, and only N₂ gas may beused as the ambient gas. On the other hand, the p-type clad layer 10 maybe omitted, and the second contact layer 21 may be directly grown on theactive layer 17. In this case, since the growth temperature of thesecond contact layer 21 is relatively higher than the growth temperatureof the active layer 17, it is necessary to raise the substratetemperature after the formation of the active layer 17. At this time, H₂gas is not supplied when the substrate temperature is being raised.

On the other hand, the transparent conductive layer 23, such as ITO orZnO, may be formed on the second contact layer 21 and come into ohmiccontact with the second contact layer 21. The second electrode 27 may beconnected to the second contact layer 21 through the transparentconductive layer 23. In addition, the first contact layer 13 may beexposed by removing the second contact layer 21, the p-type clad layer19, the active layer 17, and a part of the superlattice layer 15 throughan etching process. The first electrode 25 may be formed on the exposedfirst contact layer 13.

FIG. 7 is a cross-sectional view for describing a light emitting diodeaccording to another embodiment of the present invention.

Referring to FIG. 7 the light emitting diode includes a gallium nitridesubstrate 111, an intermediate-temperature buffer layer 13, a firstcontact layer 119, a superlattice layer 120, an active layer 130, and asecond contact layer 143. In addition, the light emitting diode mayfurther include a clad layer 141, a transparent electrode layer 145, afirst electrode 147, and a second electrode 149.

The gallium nitride substrate 111 may have an m-plane or a-planenonpolar growth surface or semipolar growth surface. In addition, thegrowth surface of the gallium nitride substrate 111 may have a tiltangle for assisting the growth of an epitaxial layer. The galliumnitride substrate 111 may be fabricated using, for example, an HVPEtechnique.

The intermediate-temperature buffer layer 113 may be formed on thegallium nitride substrate 111. The intermediate-temperature buffer layer113 may be formed to have a thickness of about 2 to 10 nm in atemperature range of about 700 to 800° C.

Conventionally, technique for forming a low-temperature buffer layer ata temperature below 600° C. has been used for growing a galliumnitride-based epitaxial layer on a sapphire substrate. Due to thelow-temperature buffer layer, the gallium nitride-based epitaxial layercan be grown on the sapphire substrate with lattice mismatch and largethermal expansion coefficient difference. However, the gallium nitridesubstrate 111 is a homogeneous substrate to the gallium nitrideepitaxial layer, and the low-temperature buffer layer is not required.Furthermore, in a case where the low-temperature buffer layer is formedon the nonpolar or semipolar gallium nitride substrate 111, thelow-temperature buffer layer is formed of an amorphous layer and is thencrystallized as the substrate temperature rises. At this time, thelow-temperature buffer layer is easily crystallized to have a c-planegrowth surface. Therefore, the method of forming the low-temperaturebuffer layer hardly grows the nonpolar or semipolar galliumnitride-based semiconductor layer with excellent crystal quality on thenonpolar or semipolar gallium nitride substrate.

On the other hand, it may be considered to directly grow a galliumnitride epitaxial layer on the gallium nitride substrate 111 at a hightemperature above 900° C. However, in a case where die gallium nitridelayer is grown at a high temperature above 900° C., the gallium nitridelayer has a very rough surface because the gallium nitride layer tendsto be grown to a c-plane. In particular, due to anisotropy of thenonpolar gallium nitride substrate, stripe-shaped patterns are easilyformed. In a case where a semiconductor device is fabricated by growingsubsequent epitaxial layers on a semiconductor layer with bad surfacemorphology, a leakage current easily occurs and nonradiativerecombination easily occurs. Thus, electrical characteristics andoptical characteristics of the semiconductor device are deteriorated.

On the contrary, the epitaxial layer grown on the buffer layer 113 canbe made to have a smooth surface by growing the buffer layer 113 at atemperature of 700 to 800° C. and growing the epitaxial layer on thebuffer layer 113.

As illustrated in FIG. 8, the substrate 111 is loaded into the chamberat room temperature Ta, and the intermediate-temperature buffer layer113 is grown during a period of time from t2 to t3 after the substratetemperature is raised to the temperature Tb of 700 to 800° C. at a rateof about 100° C./min during the period of time from t1 to t2. At thistime, N₂ gas and NH₃ gas are continuously supplied from the time whenthe temperature is raised, and TMG being the gallium source is suppliedat t2. Therefore, the GaN intermediate-temperature buffer layer 119 isformed during the period of time from t2 to t3.

The n-type contact layer 119 may be formed of Si-doped GaN. The n-typecontact layer 110 is grown on the intermediate-temperature buffer layer113, and the first electrode 147 comes into ohmic contact with then-type contact layer 119.

As illustrated in FIG. 8, the n-type contact layer 119 is grown at atemperature Tc. The temperature Tc may be above 900° C., for example,950° C. to 1,050° C., in particular, about 1,000° C. For example, afterthe intermediate-temperature buffer layer 113 is formed at thetemperature Tb, the supply of the gallium source is stopped. Thesubstrate temperature is raised at the rate of about 100° C./min duringthe period of time from t3 to t4. When the substrate temperature reachesthe temperature Tc, the n-type contact layer 119 may be grown bysupplying the gallium source again at time t4.

Preferably, after the substrate temperature reaches Tc, theintermediate-temperature buffer layer 113 maybe maintained at thetemperature Tc for a predetermined time, for example, during a period oftime from t4 to t5. For example, the intermediate-temperature bufferlayer 113 may be maintained at the temperature Tc for 3 to 10 minutes,preferably, 5 to 10 minutes. Subsequently, the n-type contact layer 110may be grown by supplying the gallium, source again at time t5. Sincethe intermediate-temperature buffer layer 113 is maintained at thetemperature Tc, the intermediate-temperature buffer layer 113 may bethermally treated and recrystallized. Accordingly, the surfacemorphology of the n-type contact layer 119 grown on theintermediate-temperature buffer layer 113 can be further improved.

Although the n-type contact layer 110 is illustrated as a single layer,the n-type contact layer 119 is not limited to the single GaN layer.Another gallium nitride-based layer may be interposed in the middle ofthe n-type contact layer 119.

On the other hand, the superlattice layer 120 with a multi-layeredstructure is disposed on the n-type contact layer 119. The superlatticelayer 120 is disposed between the n-type contact layer 119 and theactive layer 130. Accordingly, the superlattice layer 120 is disposed onan electric current path. The superlattice layer 120 may be formed byrepetitively laminating a pair of InGaN/GaN at multiple periods (forexample, 15 to 20 periods), but is not limited thereto. For example, asillustrated in FIG. 9, the superlattice layer 120 may be configured insuch a way that a three-layer structure of InGaN layer 121/AlGaN layer122/GaN layer 123 is repetitively laminated at multiple periods (forexample, 10 to 20 periods). The order of the AlGaN layer 122 and theInGaN layer 121 may be reversed. In this case, the InGaN layer 121 has awide band gap as compared with the well layer inside the active layer130. In addition, it is preferable that the AlGaN layer 122 has a wideband gap as compared with the barrier layer inside the active layer 130.Furthermore, the InGaN layer 121 and the AlGaN layer 122 may be formedof an undoped layer into which no impurity is intentionally doped, andthe GaN layer 123 may be formed of a Si-doped layer. It is preferablethat the uppermost layer of the superlattice layer 120 is animpurity-doped GaN layer 123.

Since the AlGaN layer 122 is included in the superlattice layer 120,holes inside the active layer 130 can be prevented from moving towardthe n-type contact layer 119, improving a radiative recombination ratewithin the active layer 130. The AlGaN layer 122 may be formed to have athickness less than 1 mm.

On the other hand, in the superlattice layer 120, since the AlGaN layer122 is formed on the InGaN layer 121, lattice mismatch, between theInGaN layer 121 and the AlGaN layer 122 is great, and thus, crystaldefect is easily formed on an interface therebetween. Therefore, asillustrated in FIG. 10, a GaN layer 124 may be inserted between theInGaN layer 121 and the AlGaN layer 122. The GaN layer 124 may be formedof an undoped layer or a Si-doped layer.

The active layer 130 with a multiple quantum well structure is disposedon the superlattice layer 120. As illustrated in FIG. 11, the activelayer 130 has a structure in which barrier layers 31 a and 31 b and welllayers 33 n, 33 and 33 p are alternately laminated. The well layer 33 nrepresents a well layer (first well layer) closest to the superlatticelayer 120 or the n-type contact layer 119, and the well layer 33 prepresents a well layer (n-th well layer) closest to the p-type cladlayer 141 or the p-type contact layer 123. FIG. 12 illustrates an energyband of the active layer 130.

Referring to FIGS. 11 and 12, (n−1) barrier layers 31 a and 31 b and(n−2) well layers 33 are alternately laminated between the well layer 33n and the well layer 33 p. The barrier layers 31 a have a largerthickness than an average thickness of the (n−1) barrier layers 31 a and31 b, and the barrier layers 31 b have a smaller thickness than theaverage thickness. In addition, as illustrated in FIGS. 11 and 12, thebarrier layers 31 a are disposed closer to the first well layer 33 n,and the barrier layers 31 b are disposed closer to the n-th well layer33 p.

Furthermore, the barrier layer 31 a may be disposed in contact with theuppermost layer of the superlattice layer 120. That is, the harrierlayer 31 a may be disposed between the superlattice layer 120 and thefirst well layer 33 n. In addition, a barrier layer 35 may be disposedon the n-th well layer 33 p. The barrier layer 35 may have a relativelylarger thickness than the barrier layer 31 a.

By relatively reducing the thicknesses of the barrier layers 31 b closeto the n-th well layer 33 p, the resistance component of the activelayer 130 can be reduced. In addition, holes injected from the p-typecontact layer 143 can be dispersed to the well layers 33 inside theactive layer 130. Therefore, the forward voltage of the light emittingdiode can be reduced. In addition, by relatively increasing thethickness of the barrier layer 35, crystal defects generated whilegrowing the active layer 130, in particular, the well layers 33 m, 33and 33 p are recovered. Therefore, crystal quality of the epitaxiallayers formed on the active layer 130 can, be improved. However, in acase where the barrier layers 31 b are formed more than the barrierlayers 31 a, defect density inside the active layer 130 may beincreased, and thus, luminous efficiency may be lowered. Therefore, ifis preferable that the barrier layers 31 a are formed more than theharrier layers 31 b.

On the other hand, the well layers 33 u, 33 and 33 p may havesubstantially the same thickness. Accordingly, light having a verynarrow full width at half maximum can be emitted. On the contrary, lighthaving a relatively wide full width at half maximum can be emitted bydifferently adjusting the thicknesses of the well layers 33 n, 33 and 33p. Furthermore, it is possible to prevent the generation of the crystaldefects by relatively reducing the thickness of the well layer 33disposed between the barrier layers 31 b as compared with the well layer33 disposed between the barrier layers 31 a. For example, the thicknessof the well layers 33 n, 33 and 33 p may be within a range from 10 to 30Å, the thickness of the barrier layers 31 a may be within a range from50 to 70 Å, and the thickness of the barrier layers 31 b may be within arange from 30 to 50 Å.

In addition, the well layers 33 n, 33 and 33 p may be formed of agallium nitride-based layer which emits near-ultraviolet light, bluelight, or green light. For example, the well layers 53 n, 33 and 33 pmay be formed of InGaN, and an In composition ratio is adjustedaccording to a required wavelength.

On the other hand, the barrier layers 31 a and 31 b are formed of agallium nitride-based layer with a wider band gap than the well layers33 n, 33 and 33 p so as to confine electrons and holes within the welllayers 33 n, 33 and 33 p. For example, the barrier layers 31 a and 31 bmay be formed of GaN, AlGaN, or AlInGaN. In particular, the barrierlayers 31 a and 31 b may be formed of an Al-containing galliumnitride-based, layer so as to further improve the band gap. In thiscase, it is preferable that the Al composition ratio within the barrierlayers 31 a and 31 b is in a range from 0 to 0.1, in particular, from0.02 to 0.05. By limiting the Al composition ratio to the above range,light output can be increased.

In addition, although not illustrated, a capping layer may be formedbetween the respective well layers 33 n, 33 and 33 p and the barrierlayers 31 a and 31 b disposed thereon. The capping layer is formed forpreventing the damage of the well layers when the temperature of thechamber is being raised so as to grow the barrier layers 31 a and 31 b.For example, the well layers 33 n, 33 and 33 p may be grown at atemperature of about 780° C., and the barrier layers 31 a and 31 b maybe grown at a tempera turn of about 800° C.

The p-type clad layer 141 is disposed on the active layer 130, and maybe formed of AlGaN. Alternatively, the p-type clad layer 141 may also beformed in a superlattice structure in which InGaN/AlGaN is repetitivelylaminated. The p-type clad layer 141 is an electron blocking layer andprevents electrons from moving to the p-type contact layer 143, leadingto an improvement in luminous efficiency.

Referring again to FIG. 7, the p-type contact layer 143 may be formed ofMg-doped GaN. The p-type contact layer 143 may be disposed on the p-typeclad layer 141. On the other hand, the transparent conductive layer 145,such as ITO or ZnO, may be formed on the p-type contact layer 143 andcome into ohmic contact with the p-type contact layer 143. The secondcontact layer 149 is electrically connected to the p-type contact layer143. The second electrode 149 may be connected to the p-type contactlayer 143 through the transparent conductive layer 145.

On the other hand, the n-type contact layer 119 may be exposed byremoving the p-type contact layer 143, the p-type clad layer 141, theactive layer 130, and a part of the superlattice layer 120 through anetching process. The first electrode 147 may be formed on the exposedn-type contact layer 119.

In this embodiment, the intermediate-temperature buffer layer 113 andthe epitaxial layers 19 to 43 grown on the gallium nitride substrate 111may be formed using an MOCVD technique, At this time, TMAl, TMGa, andTMIn may be used as sources of Al, Ga and In, and NH₃ may be used as asource of N. In addition, SiH₄ may be used, as a source of Si, which isan n-type impurity, and Cp₂Mg may be used as a source of Mg, which is ap-type impurity.

Experimental Example

FIGS. 13A to 13G are optical photographs showing surface morphologies ofthe epitaxial layer according to the use of the intermediate-temperaturebuffer layer 113. FIG. 13A is a photograph showing the surface of then-type GaN layer grown, on the gallium nitride substrate 111, withoutthe intermediate-temperature buffer layer 113. FIG. 13B is a photographshowing the surface of the n-type GaN layer immediately grown when theintermediate-temperature buffer layer 113 was formed on the galliumnitride substrate 111, the substrate temperature was raised to thegrowth temperature of the n-type GaN layer and then reached the growthtemperature of the n-type GaN layer. FIG. 13C is a photograph showingthe surface of the n-type GaN layer grown when theintermediate-temperature buffer layer 113 was formed on the galliumnitride substrate 111, the substrate temperature was raised to thegrowth temperature of the n-type GaN layer and then theintermediate-temperature buffer layer was maintained at the growthtemperature of the n-type GaN layer for 5 minutes.

A substrate with an m-plane growth surface was used as the galliumnitride substrate 111. The intermediate-temperature buffer layer 113 wasformed to have a thickness of about 5 nm at a temperature of about 750°C. when a ratio of NH₃ to TMG, that is, a V/III ratio is set to about357.1. The n-type GaN layers were all grown at 1,000° C. when the V/IIIratio was set to about 76.9.

Referring to FIG. 13A, in a case where the n-type GaN layer was directlygrown without forming the intermediate-temperature buffer layer 113, itcan be seen that the surface of the n-type GaN layer was very rough.Since the crystal orientation of the GaN layer grown on the galliumnitride substrate 111 is locally changed, the epitaxial layer has arough surface. In addition, the stripe-shaped surface morphology isobserved. This is determined as a phenomenon occurring because thegallium nitride substrate 111 has anisotropy.

On the contrary, referring to FIG. 13B, it can be seen that the n-typeGaN layer having a smooth surface was formed by the use of theintermediate-temperature buffer layer 113. That is, since theintermediate-temperature buffer layer 113 alleviates the surface defectsof the growth substrate 111, the crystal quality of the epitaxial layerformed thereon is improved.

Therefore, it can be seen that the crystal quality of the epitaxiallayer grown on the intermediate-temperature buffer layer 113 at a hightemperature of 900° C. can be improved by growing theintermediate-temperature buffer layer 113 at a temperature of 700 to800° C.

Furthermore, referring to FIG. 13C, it can be seen that the surfacemorphology of the n-type GaN layer is further improved by maintainingthe intermediate-temperature buffer layer 113 at the growth temperatureof the n-type GaN layer for a predetermined time.

In the foregoing embodiments, the light emitting diode has beendescribed for illustrative purpose, but the present invention is notlimited to the light emitting diode. The present invention can also beapplied to any type of a semiconductor device adopting a nonpolar orsemipolar gallium nitride-based semiconductor layer.

According to the embodiments of the present invention, the nonpolargallium nitride layer with excellent surface morphology can be grown onthe nonpolar gallium nitride substrate. Therefore, the semiconductorlayers with excellent crystal quality can be grown on the nonpolargallium nitride substrate, and the semiconductor devices, such as thelight emitting diode with high luminous efficiency, can be fabricatedusing these semiconductor layers. In addition, by adopting theintermediate-temperature buffer layer, the nonpolar or semipolargallium, nitride-based semiconductor layers with excellent crystalquality can be grown on the nonpolar or semipolar gallium nitridesubstrate. The semiconductor devices can be provided using the nonpolaror semipolar gallium nitride-based semiconductor layers grown on theintermediate-temperature buffer layer. In particular, the nonpolar orsemipolar light emitting diodes with improved luminous efficiency can beprovided.

While the embodiments of the present invention have been described withreference to the specific embodiments, it will be apparent to thoseskilled in the art that various changes and modifications may be madewithout departing from the spirit and scope of the invention as definedin the following claims.

1-21. (canceled)
 22. A method of fabricating a nonpolar gallium nitridesemiconductor device using metal organic chemical vapor deposition, themethod comprising: growing an intermediate-temperature buffer layer overa nonpolar gallium nitride substrate at a growth temperature of a rangefrom 700° C. to 800° C.; and growing a gallium nitride based firstsemiconductor layer over the intermediate-temperature buffer layer at agrowth temperature above 900° C.
 23. The method of claim 22, wherein thenonpolar gallium nitride substrate has an m-plane growth surface, andthe intermediate-temperature buffer layer is grown on the m-plane growthsurface.
 24. The method of claim 23, wherein theintermediate-temperature buffer layer includes a GaN layer.
 25. Themethod of claim 22, further including: forming an active layer over thefirst semiconductor layer; and forming a second semiconductor layer overthe active layer.
 26. The method of claim 22, wherein the nonpolargallium nitride semiconductor device includes a nonpolar light emittingdiode.
 27. The method of claim 25, further including: forming asuperlattice layer with a multi-layered structure between the firstsemiconductor layer and the active layer.
 28. The method of claim 25,wherein the active layer includes a structure in which barrier layersand well layers are alternately laminated, and wherein the barrierlayers include a first barrier layer, a second barrier layer, and athird barrier layer which have different thicknesses from one another.29. The method of claim 28, wherein the first barrier layer has agreater thickness than an average thickness of the barrier layers. 30.The method of claim 29, wherein the first barrier layer is disposed at abottommost position in the structure of the active layer.
 31. The methodof claim 29, wherein the thickness of the first barrier layer includes athickness from 50 Å to 70 Å.
 32. The method of claim 29, wherein thethird barrier layer has a greater thickness than the thickness of thefirst barrier layer, and is disposed at a topmost position in thestructure of the active layer.
 33. The method of claim 28, wherein thesecond barrier layer has a smaller thickness than an average thicknessof the barrier layers.
 34. The method of claim 33, wherein the secondbarrier layer is disposed between a topmost position in the structure ofthe active layer and a bottommost position in the structure of theactive layer.
 35. The method of claim 33, wherein the thickness of thesecond barrier layer includes a thickness from 30 Å to 50 Å.
 36. Themethod of claim 28, wherein the first barrier layer has a greaterthickness than that of the second barrier layer, the barrier layersinclude multiple first barrier layers and multiple second barrierlayers, and a number of the first barrier layer is greater than a numberof the second barrier layer.
 37. The method of claim 28, wherein thewell layers have substantially the same thickness.
 38. A method offabricating a nonpolar gallium nitride semiconductor device using metalorganic chemical vapor deposition, the method comprising: providing agallium nitride substrate; growing an intermediate-temperature bufferlayer over the gallium nitride substrate in a first temperature rangehigher than 600° C.; growing an n-type contact layer over theintermediate-temperature buffer layer in a second temperature rangehigher than the first temperature range; and growing an active layerover the n-type contact layer to have a structure in which barrierlayers and well layers are alternately laminated, and wherein thebarrier layers have varying thickness from one surface of the activelayer to the opposite surface of the layer.
 39. The method of claim 38,wherein the first temperature range is from 700° C. to 800° C. and thesecond temperature range is above 900° C.
 40. The method of claim 38,wherein the barrier layer that is positioned close to the n-type contactlayer has a greater thickness than an average thickness of the barrierlayers.
 41. The method of claim 38, wherein the barrier layer that ispositioned far from the n-type contact layer has a smaller thicknessthan an average thickness of the barrier layers.
 42. The method of claim38, further comprising: after the growing of the n-type contact layerand before the growing of the active layer, growing a superlattice layerover the n-type contact layer.